ABSTRACT:
The attenuation property of Ebony wood and Perspex, as well as the radiographic contrast produced by these materials, were studied along side those of aluminium, tin and copper metals (for comparison) at the x-ray energy of 67kV and 40mAs, to assess the possibility of the adoption of Ebony wood and Perspex as x-ray cassette window materials. Results obtained revealed that ebony wood and Perspex attenuated the beam moderately (42.3% and 47.4%, respectively) and recorded radiographic contrast values above 0.5 (50% of optimum contrast in radiology), higher than values obtained for tin and copper, but less than contrast values recorded with aluminium. The results suggest the possibility of the use of ebony wood and Perspex, probably with treatment for enhancement, as x-ray cassette window materials, in addition to their use as shielding materials at much lower energies.Keywords: Attenuation, Ebony wood, Perspex, Cassette, Contrast
INTRODUCTION :
Radiographic image quality is a primary factor in the utility of a radiograph for diagnostic purposes. The image must contain certain diagnostic information in a form that is easy to extract visually. The ease with which this information is extracted is dependent on the magnitude of the densities and density difference (Jenkins,1981) recorded on the radiograph.
Such densities are themselves heavily dependent on several factors, like exposure factors, thickness of the part irradiated, the attenuation properties of the cassette front radiolucent window, among others (Christensen et al, 1978). The attenuation properties of a material determine its attenuation coefficient, which is a measure of the quantity of radiation attenuated by a unit thickness of that material traversed (Christensen et al, 1978; Hay & Hughes, 1978; Ekpe & Essien, 1999). Different materials have different attenuation properties because of different factors which include the nature of radiation, the properties of the medium - atomic number, density and electrons per cubic centimeter (Mescham & OH, 1984 ; Brant and Helms, 1998).
The quality of the transmitted beam, is therefore dependent on the attenuation properties of the traversed material and determines the quality of the radiographic image, which is formed on a light sensitive medium, the x-ray film (Jenkins, 1981; Chesney & Chesney, 1981). Materials used in x-ray cassette manufacturing are made into light tight containers and are made to have a uniform radiolucent cassette front (Jenkins, 1981; Egbe, 2004), which serves as the window for x-ray transmission to reach the recording medium. Several materials have been used for this purpose some of which have already been dropped. However research into the possible utility of some other materials is needful, as the search for optimum benefits in image quality (Corr, 2001) and radiation protection (Munro, 2004) continues. Usually low attenuating materials are utilized in the making of x-ray cassettes since much of the radiation beam needs to reach the detector (film) to record the image. Poor film contrast, as one of the determinants of image quality, will introduce the necessity of repeated radiograph with the attendant undesirable effects on patient dose increase.
This work focuses principally on the attenuation of wood (Ebony) and Perspex used as cassette front (window) materials, side by side with aluminium, copper and tin, with a view to possible adoption of these materials for further development as cassette front materials, particularly in places where these materials exist in abundance.
MATERIALS AND METHODS :
Cassettes were constructed using the materials under study - aluminum, copper, tin and Perspex and wood. Construction was such that the cassette window, through which x-radiation passes into the cassettes to reach the film, was made of a thin sheet of the test material (2.0 mm thickness) and calcium tungstate based intensifying screens (which emit blue) attached to the back of the window material. Screen type films of the Konica media were placed within the boxes, which had provision for being locked. The experimental cassettes were then severally and individually exposed to x-radiation from a high frequency single-phase x-ray generator having a maximum output of 150kVp, 500mA and 0.5 seconds, and a minimum output of 45kVp, 50mA and 0.001 seconds, respectively. Exposures were made at 67kVp, 200mA and 0.2 seconds (40mAs), with the selection of factors based on the variable kV method for adult mid-thigh measurement of 20cm, described by Munro, (2004). The experimental set up is as shown in Figure 1, below.
Bone and water phantom materials (dry pelvic bone and a sachet of water in a cellophane bag) were placed above the cassette front window being tested, for determination of contrast (image quality). One thermoluminiscent dosimeter (LiF) chip each, labeled as TLD A and TLD B, respectively, were placed above and immediately below the cassette window to measure the "skin dose" (above) and the transmitted intensity (below), of the x-ray beam, respectively. The TLD chips were read with a vinten solaro TLD reader calibrated in µSv, and the attenuated components of the beam for each material was computed using Equation 1, below :
TLD A - TLD B = Attenuated quantity (Eq. 1)
The exposed films were processed manually for 3 minutes, in a developer bath with temperature 26oC, then fixed and washed for 10 and 30 minutes respectively. Processed films were dried in a separate dryer (Holm, 2000; Lloyd, 2001) and the recorded densities read with a Sakura model PDA - 85 digital densitometer From these, contrast between the bone and the water phantom determined for assessment of the radiographic contrast by Equation 2, below :
Dw - Db = Contrast (Eq. 2)
where Dw and Db represent the density readings for bone and water phantom, from the densitometer.
The attenuation coefficients were determined from the mathematical relationship expressed in Equation 3 (Hay & Hughes, 1978; Ekpe & Essien, 1999; Inyang et al, 1999).
I = Io e-µx (Eq. 3)
All the results obtained are presented in Table 1 below.
Table 1: Transmission of the x-ray beam by different window materials
AttenuationAluminium
DISCUSSION :
Usually low attenuating materials are utilized in the making of x-ray cassette front since much of the primary beam is required to reach the detector - x-ray film, to form the image. However, the attenuation of the primary beam is usually not altogether due to absorption by the traversed material, but is equally dependent on the scattered component of the beam (Ching-Shen & Shyi-Ming, 1996; Ekpe & Essien, 1999). Transmitted intensity is dependent on the density and the thickness of the traversed material, as well as other factors, which also include the energy of the beam (Ekpe & Essien, 1999). The absorbed component is due to energy losses within the material as a result of interactions between the radiation and the atoms in the material (Ching-Shen & Shyi-Ming, 1996; Ekpe & Essien, 1999). The magnitude of the aluminium equivalent of the material is a determinant of the image contrast (Egbe, 2004).
Our results show that the materials used in this study as cassette window for x-ray transmission, showed relatively moderate attenuation of the beam at the energy used. The attenuation properties of Perspex and Ebony wood present some interesting results, suggesting the possibility of their use as radiation shielding materials at low energies. Poor radiographic contrast, as one of the parameters used in assessing image quality, usually will introduce the necessity for repeated radiographs with the attendant undesirable effect on patient dose. This is one reason low attenuating materials that combine production of an image of good diagnostic quality (having acceptable contrast) are preferred for diagnostic radiography.
Images produced by the different materials studied of the bone and water phantoms showed long scale contrast for tin and copper, with significant shades of grey, while the image produced by the aluminium window showed short scale contrast (large difference in densities recorded) (Munro, 2004). Perspex and wood recorded appreciable contrast - somewhere between the two scales. This suggests that Perspex and wood, perhaps with proper treatment, could be used for making specialized cassettes for radiographic examinations requiring mid-scale contrast images.
CONCLUSION :
From the foregoing, Ebony wood and Perspex could be useful as x-ray cassette front materials at energies of less or equal to 67kV.
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